1. Field of the Invention
The present invention generally relates to a valve metal anode for a capacitor, and more particularly, to an electrolytic capacitor comprising an anode formed from a pressed pellet of tantalum fibers. The tantalum fiber pellet is sintered and then anodized into a high voltage anode at formation voltages up to 550V.
2. Prior Art
Development of powders suitable for making a tantalum capacitor has been a focus of both capacitor producers and tantalum processors. Historically, the intent has been to delineate requirements for tantalum powder that will result in capacitors having reliable performance, particularly in demanding high voltage applications such as cardiac defibrillation. It is understood that demanding applications, such as cardiac defibrillation, require tantalum powders having suitable surface area, high purity, uniform feature size, optimized shrinkage, favorable flowability and pressability, and green pellet strength.
Wet tantalum capacitors have been used in implantable cardiac defibrillators as the output energy storage capacitor for delivering the therapeutic electrical shock to the heart to stop a defibrillation event. These shocks are generally delivered at voltages ranging from approximately 650 volts to 950 volts. To achieve therapy delivery at such high voltage levels, between three and four tantalum capacitors are typically used in the output stage of the defibrillator.
Several advantages are associated with reducing the number of capacitors. For example, fewer capacitors required for energy storage simplifies the device charge and discharge circuits. Also, a reduction in the number of capacitors results in a reduction in the number of components in the device. Fewer components mean that the potential for performance issues decreases, thereby favorably impacting reliability. Other advantages of fewer components are more efficient assembly and lower cost.
Accordingly, one purpose of this invention is to develop a manufacturing process for tantalum anodes that are suitable for building an electrolytic capacitor for incorporation into a cardiac defibrillator. The manufacturing processes include pressing, sintering and forming steps. It is also the purpose of this invention to fabricate a tantalum anode that is capable of being formed at higher voltages than is currently known in the prior art. An anode for high voltage applications such as described within must also have a pore structure and internal surface area that allows for low ESR and high capacitance.
It is known in the art that ESR is related to energy loss. It is also known that for a capacitor, energy loss during charging and discharging impacts capacitor efficiency. Hence, a lower ESR of an anode made in accordance with the present invention improves the efficiency of the capacitor. This is of significance in cardiac defibrillation as discharge of the capacitor delivers the energy needed to return the heart to normal rhythm. The improved efficiency achieved by the present invention enables delivery of energy and higher voltages, and allows for smaller batteries to be used in implantable defibrillators due to less energy being required to charge the capacitors. Improvement in the capacitance per unit volume of an anode of the present invention allows more charge to be stored per unit volume, resulting in a capacitor that stores more energy per unit volume.
When tantalum powders are formed into a porous anode body and then sintered for use in an electrolytic capacitor, it is known that the resultant anode capacitance is proportional to the specific surface area of the sintered porous body. The greater the specific surface area after sintering, the greater the anode capacitance (μFV/g) is. Since the anode capacitance (μFV/g) of a tantalum pellet is a function of the specific surface area of the sintered powder, one way to achieve a greater net surface area is by increasing the quantity (grams) of powder per pellet. However, with this approach cost and size increase considerably. Consequently, cost and size considerations dictate that tantalum powder development focus on means to increase the specific surface area of the powder itself.
Prior art methods for increasing the specific surface area of tantalum powder include flattening the powder particles into a flake shape or spherical granulation to produce ovular particle shapes. For example, U.S. Pat. No. 4,940,490 to Fife et al., U.S. Pat. No. 5,211,741 to Fife and U.S. Pat. No. 5,580,367 to Fife disclose flaked tantalum powders and methods for making the flaked powders. FIG. 1 is an SEM photograph at 5,000× showing flake tantalum particles according to the prior art.
However, efforts to further increase specific surface area by making thinner tantalum flakes have been hindered by concomitant loss of processing characteristics. For example, several of the major deficiencies of very thin tantalum flake are poor flow characteristics, poor pressability and low green strength, and low forming voltages. Moreover, increasing particle size using spherical granulation still tends to result in particles that are finer than desirable. The resultant pore size and structure of pellets made from spherical particles tend to be smaller. Pellet structure damage during high temperature formation is a further area of concern.
One commonly used tantalum powder having relatively large particles is commercially available from H. C. Starck under the designation QR-3. This so called EB melt-type tantalum powder permits anodes to be made with relatively larger pore structures. The larger pore structures allow formation electrolytes to cool the interior of the pellets during formation. However, the relatively small surface area of these large particle size powders does not result in anodes of high capacitance per unit volume. That is because the relatively large particle size results in excessive amounts of tantalum metal remaining after formation of tantalum oxide. FIG. 2 is an SEM photograph at 1,000× showing EB melt tantalum particles according to the prior art.
Another commonly used material is available from H. C. Starck as sodium reduced tantalum powder under the designation NH-175. Because of its relatively higher surface area, this material is known to produce anodes with higher capacitance than QR-3 powders. However, because of its smaller feature size and broad particle size distribution, NH-175 powders are also known to produce anodes with smaller pore structures. The smaller pore structure makes internal cooling of anode pellets during anodization more difficult, and limits the formation voltages that these anodes can achieve. If formation voltage gets too high, many of the NH-175 tantalum particles are formed completely through, leaving no conductive pathways behind the tantalum oxide. FIG. 3 is an SEM photograph at 5,000× showing a sodium reduced NH-175 tantalum powder agglomerate according to the prior art.
Purity of the powder is another important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in tantalum capacitors. While high sintering temperatures serve to remove some volatile contaminants, not all may be removed sufficiently, resulting in sites having high DC leakage. High DC leakage is known to contribute to premature electrical failures, particularly in high voltage applications. Further, high sintering temperatures tend to shrink the porous anode body, thereby reducing its net specific surface area and thus the capacitance of the resulting capacitor. Therefore, minimizing loss of specific surface area under sintering conditions, i.e., shrinkage, is necessary in order to produce high μFV/g tantalum capacitors.
Flowability of tantalum powder and green strength (mechanical strength of pressed, unsintered powder pellets) are also important characteristics for a capacitor producer. Not only does flowability provide for efficient pellet production, it provides for high volume, automated pellet production. Flowability of agglomerated tantalum powder is even more essential to production efficiency and proper operation of automatic pellet presses. Sufficient green strength permits handling and transport of a pressed product, e.g., pellet, without excessive breakage or pellet damage (detectable and undetectable) that could affect production reject rates and finished product performance.
Accordingly, what is needed is a tantalum fiber of a strictly controlled diameter such that sufficient metal remains after formation to provide a conductive matrix behind the dielectric oxide. Because of the tightly controlled fiber diameter according to the present invention, fiber diameter can be minimized to a greater extent than with other prior art powder types. By minimizing fiber diameter while ensuring that tantalum is not totally consumed during formation, the dielectric surface area can be maximized without isolating dielectric area due to loss of tantalum substrate.
In that respect, a tantalum anode according to the present invention is distinguishable from the prior art. Regardless whether the tantalum is of a flake or spherical shape manufactured by the beam melt (QR-3 powder) or sodium reduction processes (NH-175 powder), the present invention uniquely discloses the pressing and sintering of an agglomerate of tantalum fibers having a tightly controlled aspect ratio. The result is an electrode pellet having a dual morphology and that is capable of being anodized into a capacitor anode at formation voltages up to 550V.